NAND flash devices, among others, rely on inter-poly dielectric (IPD) stacks comprising oxide and nitride layers for data retention. As device scaling has surpassed the 45 nm node, engineering of IPD stacks has garnered increased interest due to data leakage, for example in floating gate NAND devices. A floating gate NAND device typically includes a floating gate with a stack of alternating nitride/oxide layers formed thereon. The layers are typically formed conformally over the device surface, with a nitrided polysilicon layer contacting the gate oxide layer.
Hydrogen can be incorporated into the nitride matrix interstitially or through a bond. The hydrogen-nitrogen bond (4 eV) or hydrogen-silicon (3.29 eV) is not easily broken using heat alone, so while a post deposited nitride anneal may drive interstitial hydrogen from the matrix, it will not be able to break the hydrogen bond. However, through the natural lifetime and cycling of a device, some of these hydrogen bonds will be broken and leave behind an unwanted trap. This results in increased leakage through the nitride, as well as unwanted hydrogen incorporation elsewhere in the film stack, which poses an increasing challenge as device geometry shrinks. In smaller geometries, the deposited nitride thickness is limited by the physical dimensions of the device, allowing increased leakage through the IPD as well as along the IPD, specifically through the nitride.
Post nitride deposition film treatment using DPN and RPN have been shown to improve the wet etch rate of the deposited nitride more than what would occur from a simple thermal anneal. However, less than 20 A is the maximum depth of improvement due to the tight Si3N4 matrix. It is believed that similar to SiO2 densification, Si3N4 is improved due to bond breaking and rearranging during ion and radical diffusion through the film. However, the rigid nature of Si3N4 acts as a natural screen for large molecules such as O2 and N2, the dominant ionic species in the local plasma generally being O2+ or N2+. Therefore only the top portion (˜20 A) is fully treated after deposition. Deeper portions are still improved due to the thermal treatment that also occurs in the case of RPN, but a drastic change in WER is observed between the two regimes. Accordingly, a method of forming nitride layers having superior WER is needed.